Moreover, the invention is concerned with a component, particularly crucible, which is present prior to its intended use at a high working temperature in a crucible drawing method for quartz glass, which component comprises a base body comprising a wall of tungsten or a tungsten-containing high temperature-resistant alloy which is covered at least in part with a protective layer which contains a coating metal selected from the group consisting of iridium, rhenium, osmium and ruthenium.
Crucible drawing methods are standard methods in making cylindrical components of quartz glass with any desired cross-section. The crucible materials used therein are normally tungsten (W), molybdenum (Mo), or alloys thereof. However, these refractory metals are not completely corrosion-resistant to molten quartz glass and react at high temperatures with oxygen from the quartz glass. In this reaction, metal oxides are formed that are enriched on the crucible wall and in the bottom area of the crucible from where they are removed from time to time with the melt flow of the glass melt in a concentrated form and are then noticed as striae or discolorations and lead to waste, just like undissolved metal oxide particles in the quartz glass melt.
Although crucibles of high-melting metals of the group iridium, rhenium, osmium and ruthenium show a much better corrosion resistance to a quartz glass melt, they are very expensive. The formation of a crucible from iridium is for example suggested in JP 02-022132 A.
An alternative would be to apply a protective layer of metals of said group only onto the inside of a crucible otherwise consisting of tungsten or molybdenum. Crucibles of such a type are for example known from U.S. Pat. Nos. 6,632,086 B1, 6,422,861 B1 and 6,739,155 B1. It is suggested therein that the inside of the crucible should be provided with protective layers of iridium, rhenium, osmium or alloys of said coating metals. The protective layer is either metallurgically connected to the crucible wall, or it forms a separate insert member that rests on the crucible wall and is mechanically fixed to said wall. The thicknesses of such protective layers range from 0.5 mm to 1.27 mm.
The last-described crucible shows a better corrosion resistance to quartz glass melts. The material costs for making the crucibles are however very high because of the expensive coating metals for making the protective layer.